My Arduino UNO R3 stopped working. The power and the yellow led light remain on. Pressing the reset button has no effect. When I plug it in via USB it does not appear and if it's powered via an external adapter and I plug the USB in, my macbook shuts down.

Does anyone know what the problem could be and what could have happened?

A capacitor is bad or the voltage regulator has shortcut or something like that.

On the Uno are two chips. One for the USB-to-serial and the main microcontroller (ATmega328P) which runs your sketch. If the yellow led stays on, the main microcontroller is not working. If the board is not recognized by the computer, the USB-to-serial part is not working. If both are not working, it could be a voltage problem.

Don't connect it to a computer anymore, but use an adapter for the tests.Can you measure the voltages on the board with a multimeter ?

Could you tell me where you'd like me put the probes? The power supply is from a decommissioned linksys WAP54G. The reading I'm getting in the power supply is 18 volts even tho it's rated 12. The board also seems to get very hot.

First of all, use a power supply with a lower voltage, say 9V (7...12V).Measure the pins "5V" and "3V3"If they are okay, measure the pins of the voltage regulator.If that is all okay, you could try to write a sketch to the ATmega328P (the main microcontroller) with a programmer and the ISCP header (the 2x3 pins near the ATmega328P). But you need a programmer for that.

I attached a 5V power source via a USB cable to a phone charger, I get 3.29V and 4.95V respectively which seems ok. The input regulator I'm not sure how it works (yes, I'm a n00b), I attach the positive probe to the pin closest to the round adapter socket and the negative probe to the other end and I'm also getting 4.95V. Does this mean that the board is salvageable?

I've placed an order for another arduino board to be the programmer. Is that you mean?

What do I use as a stable power supply to get the rated voltage? Battery is not feasible as I'll also be using a GSM shield which will drain it out in no time.

Buying another Arduino board is good, in case this one is damaged.You can use it to (re)program the bootloader or use it to upload a sketch via the ISCP header.I use an USBasp programmer myself (with an adapter from 10-pin to 6-pin header).

If the voltages are okay, and everything else is wrong, perhaps the microcontrollers are broken. But in many cases they still can be repaired.If you have not the official Arduino Uno, but a cheap version from Ebay, the 16MHz could be a very cheap one and get easily broken.

Just an update, I tried plugging the 12V adapter in to the arduino and measured the voltage, I got a reading of 9.7 volts. When I tested the 5V pin, I got a reading of 8.6 Volts. Maybe this is causing a problem?

It seems that the Arduino I purchased was a fake, I wrote a blog post on the topic here http://www.mishari.net/2013/01/fake-arduino-sold-on-thai-sites/ comparing the two. I hope this can serve as a warning for somebody. :-(

Sometimes I buy those from Ebay. Those boards use the cheapest components.One had bad soldering, another had a defect crystal. And an Arduino Uno uses a ATmega328 instead of a ATmega328P, which causes a lot of problems, since Arduino expects the bootloader of a ATmega328P. But in the end I was able to repair them all.

The cheapest crystals might stop working if the temperature is below 0 degrees Celsius (freezing point).So for any serious application the original is much better.